Singular perturbations and Lindblad-Kossakowski differential equations

TL;DR

This paper develops a systematic method to simplify the dynamics of quantum systems with fast and slow processes, preserving key properties, and demonstrates its effectiveness through numerical simulations on a 5-level system.

Contribution

It introduces a novel adiabatic reduction technique for Lindblad-Kossakowski equations in quantum systems with measurement-induced decoherence, maintaining the Lindblad form.

Findings

The reduced equations retain the Lindblad structure.

The method accurately approximates the system dynamics.

Numerical simulations confirm the validity for a 5-level system.

Abstract

We consider an ensemble of quantum systems whose average evolution is described by a density matrix, solution of a Lindblad-Kossakowski differential equation. We focus on the special case where the decoherence is only due to a highly unstable excited state and where the spontaneously emitted photons are measured by a photo-detector. We propose a systematic method to eliminate the fast and asymptotically stable dynamics associated to the excited state in order to obtain another differential equation for the slow part. We show that this slow differential equation is still of Lindblad-Kossakowski type, that the decoherence terms and the measured output depend explicitly on the amplitudes of quasi-resonant applied field, i.e., the control. Beside a rigorous proof of the slow/fast (adiabatic) reduction based on singular perturbation theory, we also provide a physical interpretation of the result in the context of coherence population trapping via dark states and decoherence-free subspaces. Numerical simulations illustrate the accuracy of the proposed approximation for a 5-level systems.